It is known to drive all of the various accessories that are mounted to the front of the engine block, such as the power steering pump, the alternator, the air conditioning compressor, and the water pump, with a single, serpentine belt, in the interests of simplicity and cost saving. While such a long belt is particularly subject to stretch over its life, it is possible to maintain continuous tension in the belt with a belt tensioner. These typically include an arm pivoted to swing about one end relative to the engine block, with an idler pulley at the other end that is forcibly pushed into the inside or outside of a run of the belt by a strong spring wrapped around the pivot. A good example of such a tensioner may be seen in the U.S. Pat. No. 4,473,362 to Thomey et al.
There cases where packaging restraints require the use of two different belts, each of which is to be driven from the crankshaft, but which would wind through differing paths. In such case each belt would require a separate tensioner. Using a single tensioner with a double grooved idler pulley would be impractical, since different belts, especially ones of different lengths, would not stretch by equal amounts. The obvious solution of using two tensioners, while workable, would double the cost of keeping the belts in tension, and would also represent a weight penalty.